Platelet derived growth factor receptor (PDGF-R) is a transmembrane receptor tyrosine kinase. Ligand binding to the receptor results in dimerization of two receptors generally leading to intermolecular phosphorylation of each receptor, commonly referred to as autophosphorylation or transphosphorylation, and activation of the receptor complex. PDGF, which is a ligand for PDGF-R, is a dimeric protein having two polypeptide chains joined by disulfide bonds. Each polypeptide is either an A chain polypeptide or a B chain polypeptide. Thus, PDGF can have either two A chains, two B chains, or an A and a B chain.
The PDGF-R consists of two isozymes .alpha. and .beta.. Both .alpha. and .beta.-containing receptors have been associated with mitogen activity, while only the .beta.-containing receptor has been associated with chemotaxis and actin reorganization (Heldin, C-H, EMBO Journal 11:4251-4259, 1992).
According to Plate et al., Laboratory Investigation 4:529-534, 1992:
PDGF is a potent growth factor for mesenchymal and neuroectodermal cells. Endothelial cells have been considered nonresponsive to PDGF, but a recent study has shown that PDGF may have a role in angiogenesis during placenta development. In addition, it has been demonstrated, that PDGFR-b is expressed in endothelial cells in inflammatory tissue and glial tumors. This suggests, that PDGF may play a role in vascular functions in pathological conditions. [Citations omitted.] PA1 The adverse effects of PDGF in certain diseases, as discussed above, make PDGF antagonists highly desirable. We and others have recently taken several approaches to develop such antagonists. Antibodies against PDGF have proven to be useful for inhibiting both autocrine stimulation in SSV-transformed cells and the atherosclerotic process that occurs after de-endothelialization of the carotid arteries of rats. Moreover, a soluble form of the PDGF receptor has been shown to bind and inactivate PDGF, and could thus be potentially useful for inhibiting PDGF action in vivo. PA1 Another approach would be to design or find agents that compete in an antagonistic manner with PDGF for receptor binding. In order to identify peptides that interfere with PDGF binding, we systematically screened peptides derived from the B-chain sequence. One peptide was found that inhibited PDGF binding and autophosphorylation of .alpha.- as well as .beta.-receptors. However, the peptide also showed some cell toxicity and further development will be necessary before peptide antagonists become useful for in vivo studies. Low molecular weight compounds that interfere with receptor binding have been described, e.g., suramin. However, suramin is not specific enough to be clinically useful as a PDGF antagonist. We recently found that another low molecular weight compound, neomycin, at high concentrations inhibited the binding of PDGF-BB to the .alpha.-receptor, but did not inhibit binding to the .beta.-receptor. This compound thus represents an antagonist that distinguishes between the two receptor types; however, its low potency makes it unsuitable for use in vivo. Hopefully, the experiences with suramin and neomycin will aid the future design of more potent and specific PDGF receptor antagonists. The design of such antagonists would be much facilitated by the elucidation of the three-dimensional structure of the PDGF-receptor complex. PA1 PDGF antagonistic activity could also be achieved by inhibition of PDGF receptor dimerization. We hypothesized that monomeric PDGF might fail to induce receptor dimerization and might thus have antagonistic activity. Since reduction of PDGF results in loss of receptor binding, we attempted to identify the interchain disulfide bonds in order to mutate these residues and hereby prevent dimerization of the ligand. This turned out to be quite difficult due to the high density of cysteine residues in PDGF. The approach that finally succeeded involved partial reduction of the PDGF molecule using a concentration of dithiothreitol that reduced only the interchain disulfide bonds, and left the intrachain bonds unaffected. By this procedure the second and fourth cysteine residues from the N-terminus were found to form the two interchain bonds in PDGF. Analysis of a PDGF B-chain mutant in which these two cysteine residues had been mutated to serine residues revealed that it retained receptor binding activity. Is it a receptor antagonist? The answer is no, in fact, the monomeric PDGF induced both receptor dimerization and autophosphorylation. This result may indicate that PDGF-induced receptor dimerization is not only a matter of forming a bridge between two receptor molecules: the dimerization may also involve a ligand-induced conformational change of the extracellular domains of the receptors which promotes receptor-receptor interactions. One possible way of achieving an antagonistic effect, which we are currently exploring, is to combine a wild-type PDGF chain with a mutated chain that does not bind to PDGF receptors but can actively prevent dimerization of receptors. [Citations omitted.] PA1 In accordance with the present invention, there is provided a method of inhibiting abnormal cell proliferation in a patient suffering from a disorder characterized by such proliferation comprising the administration to a patient of an EGF and/or PDGF receptor inhibiting effective amount of a bis mono- and/or bicyclic aryl and/or heteroaryl compound exhibiting protein tyrosine kinase inhibition activity wherein each aryl and/or heteroaryl group is a ring system containing 0-4 hetero atoms, said compound being optionally substituted or polysubstituted.
Heldin, supra, describes the relationship of PDGF and its receptor, and discusses the role of PDGF in cancer, noting that some cancers do not produce PDGF and have central necroses. Heldin states:
Spada A. P., et al., entitled "Bis Mono- and Bicyclic Aryl and Heteroaryl Compounds Which Inhibit EGF and/or PDGF Receptor Tyrosine Kinase," PCT/US92/03736, mentions the use of certain bis mono and bicylic aryl compounds. According to Spada: